The present invention relates generally to switched-capacitor modulators and more specifically to switched capacitor modulators utilized in systems employing quadrature modulation.
Modulators have been implemented by previous designs using various types of nonlinear devices such as vacuum tubes, diodes, transistors and switches in combination with either transformers or amplifiers. A modulator of this type has been disclosed in U.S. Pat. No. 3,937,882 (Bingham) issued Feb. 10, 1976. In this type of device, if the application criteria require all spurious outputs of the modulator to be suppressed to a low level, individual adjustment of circuit parameters is necessary.
It is very desirable to have electronic network transfer functions realized in designs using only those components which can be fabricated using large scale integration techniques. One set of such components comprises switches, capacitors and operational amplifiers. The technology for using these components has become known as the switched-capacitor technology and has been described in many publications including Hosticka et al., IEEE Journal of Solid State Circuits, December, 1977, page 600.
In that publication a second order switched-capacitor filter utilizing positive and negative integrator units is disclosed.
In the development of this technology it has been noted that the transfer functions of switched-capacitor circuits are sensitive to stray capacitances from each plate of the capacitor to the substrate which is equivalent to signal ground. The larger of these stray capacitances, otherwise known as parasitic capacitances, occurs between the bottom plate of the capacitor and the substrate. This capacitance can generally be rendered harmless by configuring the circuit so that the bottom plate of the capacitor is connected to ground. This configuration will still exhibit circuit sensitivity to the smaller of the parasitic capacitances occurring between the top plate of the capacitor and the grounded substrate.
Two implementations of switched-capacitor integrators, which are completely insensitive to parasitic capacitances, are described by Martin and Sedra in Electronics Letters, June 21, 1979, page 365. A complementary pair of inverting and noninverting integrators is disclosed by Martin and Sedra as well as circuit implementations for various filter sections.
In co-pending application, Ser. No. 119,374, filed Feb. 7, 1980 in the name of the inventor of the present invention, now U.S. Pat. No. 4,295,105, switched-capacitor modulator is disclosed wherein a modulating signal is applied to an integrator which is alternately operated in the inverting and noninverting mode under the control of a carrier signal.
In systems employing quadrature modulation it is necessary to effect the simultaneous modulation of two carrier signals which are at the same frequency but separated in phase by ninety degrees.
In some applications of a switched-capacitor modulator in an amplitude modulation communication system it is desirable to use a shaped carrier signal to suppress third and fifth harmonics of the carrier. In using a shaped carrier in a switched-capacitor modulator, two capacitors are needed and a problem of balancing exists.